This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
3GPP third generation partnership project
ADC analog to digital converter
ARFCN absolute radio frequency channel number
BCH broadcast channel
BS basestation
BW bandwidth
CA carrier aggregation
CC component carrier
CDM code division multiplexing
CE channel element
CRC cyclic redundancy check
CRS common reference signal
DCF distributed coordination function
DCI downlink control information
DIFS DCF inter-frame space
DL downlink (eNB towards UE)
DRX discontinuous reception
eNB enhanced Node B. Name for Node B in LTE
eNB E-UTRAN Node B (evolved Node B)
EPC evolved packet core
E-UTRAN evolved UTRAN (LTE)
FFT fast Fourier transform
FSVB fast synchronization verification block
GPS global positioning system
HARQ hybrid automatic repeat request
IMT-A international mobile telephony-advanced
ITU international telecommunication union
ITU-R ITU radiocommunication sector
LTE long term evolution of UTRAN (E-UTRAN)
LTE-A LTE advanced
MAC medium access control (layer 2, L2)
MIB master information block
MM/MME mobility management/mobility management entity
NACK not acknowledge/negative acknowledge
Node B base station
OFDM orthogonal frequency division multiplexing
OFDMA orthogonal frequency division multiple access
OS OFDM symbol
PS-SCH primary/secondary synchronization channel
P-BCH physical broadcast channel
PCC primary cell carrier
PCFICH physical control format indicator channel
PDCP packet data convergence protocol
PDSCH physical downlink shared channel
PHY physical (layer 1, L1)
PRB physical resource block
P-SCH primary synchronization channel
RF radio frequency
RLC radio link control
RRC radio resource control
RRH remote radio head
RRM radio resource management
S GW serving gateway
SC FDMA single carrier, frequency division multiple access
SCC secondary cell carrier
SCell secondary cell
SCH synchronization channel
SFN subframe number
SI system information
SIB system information block
SIB1 SIB type 1
SINR signal to interference and noise ratio
S-SCH secondary synchronization channel
TAE timing alignment error
TCC tracking component carrier
UE user equipment, such as a mobile station or mobile terminal
UL uplink (UE towards eNB)
UTRAN universal terrestrial radio access network
WLAN wireless local access network
Of particular interest herein are the further releases of 3GPP LTE (e.g., LTE Rel-10) targeted towards future IMT-A systems, referred to herein for convenience simply as LTE-Advanced (LTE-A). Reference in this regard may be made to 3GPP TR 36.913, V8.0.1 (2009 03), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Requirements for Further Advancements for E UTRA (LTE-Advanced) (Release 8). A goal of LTE-A is to provide significantly enhanced services by means of higher data rates and lower latency with reduced cost. LTE-A is directed toward extending and optimizing the 3GPP LTE Rel-8 radio access technologies to provide higher data rates at very low cost. LTE-A will most likely be part of LIE Rel-10. LTE-A will be a more optimized radio system fulfilling the ITU-R requirements for IMT-A while maintaining backward compatibility with LTE Rel-8. Reference is further made to a Release 9 version of 3GPP TR 36.913, V9.0.0 (2009-12). Reference is also made to a Release 10 version of 3GPP TR 36.913, V10.0.0 (2011-06).
As is specified in 3GPP TR 36.913, LTE-A should operate in spectrum allocations of different sizes, including wider spectrum allocations than those of Rel-8 LTE (e.g., up to 100 MHz) to achieve the peak data rate of 100 Mbit/s for high mobility and 1 Gbit/s for low mobility. It has been agreed that carrier aggregation is to be considered for LTE-A in order to support bandwidths larger than 20 MHz. Carrier aggregation (CA), where two or more component carriers (CCs) are aggregated, is considered for LTE-A in order to support transmission bandwidths larger than 20 MHz. The carrier aggregation could be contiguous or non-contiguous. This technique, as a bandwidth extension, can provide significant gains in terms of peak data rate and cell throughput as compared to non-aggregated operation as in LTE Rel-8.
A LTE-A terminal with reception capability beyond 20 MHz can simultaneously receive transmissions on multiple component carriers. A LTE Rel-8 terminal can receive transmissions on a single component carrier only, provided that the structure of the component carrier follows the Rel-8 specifications. Moreover, it is required that LTE-A should be backwards compatible with Rel-8 LTE in the sense that a Rel-8 LTE terminal should be operable in the LTE-A system, and that a LTE-A terminal should be operable in a Rel-8 LTE system.
FIG. 1 shows an example of the carrier aggregation, where M Rel-8 component carriers are combined together to form M×Rel-8 BW (e.g., 5×20 MHz=100 MHz given M=5). Rel-8 terminals receive/transmit on one component carrier, whereas LTE-A terminals may receive/transmit on multiple component carriers simultaneously to achieve higher throughputs through bandwidths.
With further regard to carrier aggregation, what is implied is that one eNB can effectively contain more than one cell on more than one CC (frequency carrier), and the eNB can utilize one (as in E-UTRAN Rel-8) or more cells (in an aggregated manner) when assigning resources and scheduling the UE.
Carrier Aggregation (CA) in LTE-Advanced extends the maximum bandwidth in the uplink (UL) or downlink (DL) directions by aggregating multiple carriers within a frequency band (intra-band CA) or across frequency bands (inter-band CA).
A primary cell carrier (PCC) using LTE technology may be configured on a LTE licensed band for primary access providing mobility, security and state management for user terminals while a secondary cell carrier (SCC) using another carrier (e.g., a carrier using WLAN technology) may be opportunistically configured/activated on an un-licensed band for secondary access to provide additional data plane transport.
Finding a free channel in an unlicensed band becomes more and more difficult, since there are many other systems that might utilize those frequencies. Potential co-existing systems may include: IEEE 802.11b/a/g/n/ac, Bluetooth, Zigbee, etc. In case the LTE system also wishes to utilize the unlicensed band, it may face the challenge of needing more agile spectrum sharing or time sharing in order to accommodate those systems. Thus, time/frequency synchronization should be carried out by the UE as quickly as possible.
When an LTE system is deployed in a licensed band, part of the downlink physical layer signaling transmission may be specified to be performed continuously, e.g., on the primary synchronization channels (P-SCH) and secondary synchronization channels (S-SCH) with a 5 ms periodicity on subframes #0 and #5. The master information block may be transmitted with a 40 ms periodicity at a single frame number (SFN) mod 4=0 in sub-frame #0 on the physical broadcast channel (P-BCH) in mid 6 physical resource blocks (PRB). In addition, a system information block Type 1 (SIB1) is transmitted with 80 ms periodicity at SFN mod 8=0 in sub-frame #5 on the physical downlink shared channel (PDSCH). All other SIBs may be transmitted periodically in system information messages in non-overlapping common-length time-domain system-information (SI) windows as configured in SIB1.
In one technique to deploy LTE system in a shared band without any modification, the LTE will occupy the spectrum all the time in order to transmit the synchronization channel, cell-specific reference signals, and system information. This may block any other system's usage, which unfairly monopolizes the channel and may raise problems with the regulatory requirements on unlicensed band. To co-exist with 802.11 systems, the LTE system may suspend all continuous transmission when a WiFi system is occupying the wireless medium on the shared spectrum and resume transmission when the wireless medium become available. The LTE system may rely on a combination of sensing WiFi activity and frequency/time sharing of the wireless medium to determine such ON-OFF transmission patterns. In contrast, turning off all eNB transmissions of the LTE system is different from a DRX concept in Rel-10 where the UE turns off reception, but the eNB still keeps on continuously transmitting.
The above technique may solve a co-existence problem between the LTE system and a WiFi system on a shared band but it may also raise another problem. The UE may lose synchronization in time and frequency since there are no continuous PS-SCH, MIB or CRS transmissions in the configured SCC. When using inter-band CA, new carrier types without PS-SCH, CRS or BCH cannot readily be considered due to the need for the UE to achieve initial synchronization and to track synchronization parameters on the configured SCC. The configured PCC cannot be used for such a purpose as the PCC and SCC are on different bands.
After the LTE eNB turns on the system again, the UE may need some time to perform a time-offset estimation and a frequency-offset estimation based on the PS-SCH. Then the UE will need to do a CRS-based channel estimation to detect a MIB on the PBCH and to perform a MIB CRC check to ensure there is no false SCH detection. The UE may then carry out timing and frequency correction before packets on the PDSCH can be received. The CRS may be used by a time and frequency synchronization tracker, referred to as a CRS tracker, to maintain synchronization. The MIB CRC check can add an average delay of 20 ms (e.g., due to 40 ms periodicity of MIB transmissions the maximum time to wait is 40 ms), while the minimum time could be within 1 ms where the UE detects the MIB is transmitted on the PS-SCH in subframe #0 at SFN mod 4=0. For cell-edge UEs, where the MIB CRC check may fail, the delay could be a multiple of 40 ms. Such delays are not acceptable especially if a typical ON-duration period may be of at least 30 ms (maximum WiFi packet size duration) and at most a few hundred ms assuming a reasonable WiFi load.
When multiple SCCs with small timing differences are configured on the DL, it may be possible to perform and maintain synchronization faster and more reliably using the multiple carriers forming a downlink synchronization group.
A DL synchronization group may be configured via RRC/MAC signaling as part of the SCC configuration for UEs. The DL synchronization group may include carriers CH1, CH2, CH3 and CH4 is illustrated in FIG. 5. The synchronization aspects of an “always on” tracking carrier concept may be challenging for the UE to cope with a relative propagation delay difference of up to 31.3 μs among the component carriers to be aggregated in inter-band non-contiguous CA. In a non-repeater case, the timing offset due to the BS time alignment is specified to be up to 1.3 μs. The time alignment is also referred to as the Timing Alignment Error (TAB) (which may be defined such that when the eNB intends to make the Tx timing on different CC aligned, a 1.3 μs difference is allowed as the maximum error). In a repeater case, there may be an additional propagation delay difference of up to 30 μs among the component carriers (e.g., when there are repeaters/RRHs).
If no RRH/repeater is used, a 1.3 μs BS time alignment and, typically, a 0.52 μs delay (due to frequency separation) may be seen. Using a sample duration, Ts (as specified in LTE) of 55 samples per timing offset which is inherited from the CRS tracker on the other carriers CH2, CH2 and CH3 during the overlapping ON periods (CH2 on T2-T5, CH3 on T5-T9 and CH4 on T3-T5) during which the configured SCC is in an OFF period.
The residual timing offset for the CRS tracker on configured SCC may be corrected, e.g., CH1, at the beginning on its next ON period (CH1 on T8-T9). The UE cannot run the CRC tracker on the configured SCC during its OFF period since the eNB suspends all transmission. This is a difference from having the UE waking up early before the end of the quiet DRX period to re-acquire time and frequency synchronization form CRS (which may be continuously transmitted by the eNB) independently from a DRX configuration of the UEs via dedicated signaling.
The repeater case is illustrated in FIG. 6 and the RRH case is illustrated in FIG. 7 (F1 in light grey, F2 in dark grey, where the transmission range of F2 is less than that of F1). A larger timing offset will be inherited by the CRS tracker, exacerbating the described problem. In the repeater/RRH cases, the timing offset will be even larger, for example, 30 μs+1.3 μs giving a timing offset of 971 samples.
Such delay may pose a problem to maintaining synchronization with CRS tracker using CRS from other carriers. If the CRS tracker is a post-FFT detector, the FFT window processing may require good coarse time offset correction (e.g. with a residual timing offset of a few samples) and good frequency offset correction (e.g. within a few hundred Hz) which may be provided by a pre-FFT PS-SCH detector followed by post-FFT MIB CRC. However, this presents a logistical problem of having the pre-FFT PS-SCH detector following the post-FFT MIB CRC.